Thermal release device



July 27, 1937. KOCH 2,088,420

THERMAL RELEASE DEVI CE Filed April 11, 1936 Fig.5 v Fig.4.

Birnetallic 37 :30 -3l 32 ITWVG'HGOT": f 5 3/ Kurt Koch,

by Wag/oz 49 Hi Attm n e y.

Patented July 27, 1937 PATENT OFFICE THERMAL RELEASE DEVICE Kurt Koch,Berlin-Eichwalde, Germany, assignor to General Electric Company, acorporation of New York Application April 11,

1936, Serial No. 73,900

In Germany July 15, 1935 10 Claims.

My invention relates to thermal release devices, more particulariy'tocurrent carrying thermal elements of the type commonly applied tothermal circuit breakers or relays, and has for an object the provisionof an inexpensive and improved thermal element of this character.

Current carrying thermal elements, such for example as bimetallicelements, when applied to relays or control devices for providingautomatic response to predetermined current conditions, are commonlygiven such a configuration that unequal current densities exist .indifferent portions of the element. Under normal current con ditions, orupon the flow of relatively light overload currents, the thermalcapacity and the thermal conductivity of the element are ordinarilysufiicient to provide a substantially uniform heating in all parts ofthe thermal element. It has been found, however, that upon the flow ofheavy overload currents such as short-circuit currents, the regions ofhigh current density are quickly heated to excessive temperatures. Theseportions of the thermal element may, in fact, reach a red or a whiteheat and the thermal element may be damaged before the'heat can beconducted away from these portions by twisting of the white hot metaldue to the electrodynamic forces exerted on the thermal element bycurrents of such magnitude.

Although not limited thereto, my invention is particularly applicable toU-shaped bimetallic elements having a slot extending inwardly from oneedge to form the U.. In such elements, the dangerous regions of highcurrent density exist adjacent the base of the slot, especially in theportions of the thermal element adjacent the slot angles. This is due tothe fact that the current tends to concentrate along the inner edges ofthe slot. When the legs of the U-shaped thermal element are secured tosuitable terminals, the current flowing through the element follows thepath of lowest resistance between the terminals, that is, the shortestpath, and this shortest path is, of course, along the inner edges of theslot.

It is a. further object of my invention, therefore, to provide-a thermalelement in which such excessive concentration of current and excessiveheating of certain portions of the element is prevented even under shortcircuit current conditions.

In carrying out my invention in one form, I provide a thermal elementhaving a configuration such that regions of high current density wouldnormally exist therein and conductor means for bridging over saidregions to prevent excessive heating thereof under heavy overloadconditions.

For a more complete understanding of my invention, reference may now behad to the drawing, in which Fig. 1 is a somewhat schematic perspectiveview of a thermal device embodying my invention applied to'a suitablecircuit breaker; Fig. 2 is a detail view of the thermal device shown in'Fig. 1; Figs. 3 and 4 are plan and side views respectively of a modifiedform of thermal device embodying my invention; and Fig. 5 is afragmentary horizontal sectional View taken along the center line of thepin 3! of Fig. 3.

Referring now to Figs. l and 2 of the drawing, I have shown my inventionin one form as applied to a bimetallic thermal element 10 arranged tocontrol the operation of a circuit breaker II. The circuit breaker H isof the type described and claimed in a copending application of WinfieldA. Atwood, Serial No. 667,- 855, filed April 25, 1933, entitled fCircuitbreakers, which application is assigned to the same assignee as thepresent invention. In the present application, only so much of thecircuit breaker is shown as is necessary for an explanation of myinvention.

As shown, the bimetallic element I0 is provided with a slot l2 extendinginwardly from one edge sogas to provide a U-shaped member, the legs ofthe U-shaped member being secured to suitable terminal members [3 andI4. It will thus be seen that the thermal element H], which controls theoperation of the circuit breaker II in a manner. to be describedhereinafter, is connected in circuit with the contacts of thecircuitbreaker so that the thermal element is heated by the current flowingthrough the circuit breaker. The current path through the circuitbreaker may be traced from the terminal l3 upwardly through one leg ofthe thermal element across the transverse portion or yoke of the thermalelement downwardly through the other leg of the thermal element throughthe terminal member M, a suitable flexible conductor l5, a movablecontact IB, a co-operating stationary contact ll, to the other terminalI8 of the circuit breaker.

' As described above, the current flowing through the thermal element I0follows the shortest path between the terminal members l3 and I4 andaccordingly is concentrated adjacent the closed end or base of the slotl2, especially adjacent the slot angles indicated by the referencenumerals the flow of current therethrough is substantially uniformthroughout the legs and the yoke portion of the thermal element. Asdescribed above, however, upon the occurrence of short circuitconditions, the concentration of current adjacent the base 0! the slotI2 is such as normally to produce excessive heating at these portions ofthe thermal element. In accordance with my invention, I, therefore,provide conductor means comprising a bar 2| formed of high electricalconductivity material which is secured to the thermal element lladjacent the base of the slot l2, as shown, so as to provide a currentconducting bridge between the legs of the thermal element. Thus, thebridging conductor 2| forms a low resistance current path adjacent thebase of the slot I2 and by-passes a large part of the current flowing inthe legs of the thermal element so as to prevent this excessiveconcentration of current and excessive heating adiacent the base 01 theslot.

When applied to a circuit breaker oi the type shown, the conducting bar2| may also serve as a latch, the lower edge of the bar 2| beingarranged to engage a latch flnger 22 when the thermal element is in theposition shown. As is more fully described in the above referred toAtwood application, the circuit breaker II is provided with anovercenter spring (not shown) which is operatively connected to a pairof switch operating links 23 connected together by a cross portion 24,the links 23 being arranged to operate the movable contact l6 betweenthe open and closed positions upon movement of the overcenter spring. Asuitable tripping spring 25, normally under compression, biases atripping member 26 and an extending portion 21 carried thereby in adirection to separate the contacts. Normally, however, the trippingmember 26 is held against movement by engagement of the latch flnger 22with the under side of the con ducting bar 2|.

It will be understood, 0! course, that when the thermal element i0 isheated, the free end of the thermal element carrying the conducting bar2| moves in a counter-clockwise direction, as viewed in Fig. 1, todisengage the bar 2| from the latch finger 22 and permit the trippingmember 28 to move under the influence oi the tripping spring 25. Theconducting bar 2| may of course, be given any suitable shape and isconnected to the thermal element II in good electrical conductingrelation therewith so as to provide a low current path between the legsof the U-shaped thermal element.

In Figs. 3 and 4, I have shown a modified form of thermal elementembodying my invention comprising a U-shaped bimetallic element 28having the base portion of the slot 29 enlarged, as shown, to provide asubstantially circular aperture 30. Arranged in the aperture 30 Iprovide a suitable pin or rivet 3| which may be secured in position byoverturning the ends thereot, as shown best in Pig. 4. The diameter oithe pin or rivet 3| is sumciently large so as intimately to engage theedges or the aperture 3. so that the pin 3|, which is iormed of high.

electrical conductivity material, such as copper,

forms a low resistance path between the legs of the thermal element inshunt circuit relation to the yoke of the thermal element. Although Ihave shown a solid rivet or pin ll, it will, of

course, be apparent that a hollow rivet or screw may equally well beemployed so long as the material of the rivet has a sufilciently highelectrical conductivity to provide a low resistance path between thelegs of the thermal element.

I have also found that in certain cases, especially in connection withvery thin bimetallic elements, it is advantageous to provide theconducting member and the engaging portions of the thermal element witha coating of highly conducting metal. Thus, in Fig. 5 a coating 32 ofhighly conducting material is shown applied to the engaging surfaces ofthe pin 3| and the element 28. This coating may be applied in any wellknown manner, such, for example, as by spraying or by electroplating.

While I have shown a particular embodiment of my invention, it will beunderstood, of course, that I do notwish to be limited thereto sincemany modifications may be made and I, therefore, contemplate by theappended claims to cover any such modifications as fall within the truespirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A thermal device comprising a current carrying bimetallic elementhaving a configuration such that regions of high current density wouldnormally exist therein under heavy overload conditions, and conductingmeans fixedly secured to said element for bridging over said regions,the conductivity of said conducting means being so related to theconductivity of said regions that the current flowing in said regions isinsuflicient to cause excessive heating of said regions of said elementunder said heavy overload conditions.

2. A'thermal device comprising a current carrying thermal element havinga configuration such that regions oi high current density would normallyexist therein upon the flow of heavy overload currents therethrough, andconducting means permanently secured to said element at said regions forbridging over said regions, the conductivity of said conducting meansbeing so related to the conductivity of said regions that the currentflowing in said regions is insufficient to cause excessive heating ofsaid element upon the flow 01' said heavy overload currentstherethrough.

3. A thermal device comprising a current carrying thermal element havinga slot extending inwardly from one edge to form a plurality of currentcarrying legs connected by a transverse portion, and conducting meanspermanently connecting said legs immediately adjacent said transverseportion in shunt circuit relation with said transverse portion, theconductivity of said conducting means being so related to theconductivity of said transverse portion that the current flowing throughsaid transverse portion is insufilcient to cause excessive heating ofsaid element by concentration of current in said element adjacent theclosed end of said slot upon the flow of heavy overload currents in saidelement.

4. A thermal device comprising a current carrying bimetallic elementhaving a slot extending inwardly from one edge to form a pair of currentcarrying legs connected by a transverse portion of said element, and aconducting bar permanently secured to said legs immediately adjacent theclosed end 01 said slot to shunt said transverse portion, theconductivity of said conducting bar being so related to the conductivity01 said transverse portion that the current flow- 7s ing through saidtransverse portion is insufficient to cause excessive heating of saidelement by concentration of current in said transverse portion adjacentthe end of said slot upon the flow of heavy overload currents in saidelement.

5. A thermal device comprising a U-shaped current carrying element, andconducting means permanently'connected to the legs of said U- shapedmember adjacent the yoke portion thereof for shunting said yoke portion,the conductivity of said conducting means being so related-to theconductivity of said yoke'portion as to prevent excessive concentrationof current in the inner edge of said yoke upon the flow of heavyoverload currents in said element.

6. A thermal device comprising a U-shaped thermal element, means forconnecting the legs of said element in an electric circuit so thatcurrent flowing in said circuit passes through one of said legs, thenthrough the yoke portion of said element,- and then through the other ofsaid legs in series, and a conductor permanently secured to said legs inshunt circuit relation with said yoke portion immediately adjacent saidyoke, the conductivity of said conductor being so related to theconductivity of said yoke portion that the current flowing through saidyoke portion is insufficient to cause excessive concentration of currentat the inner edge of said yoke upon the flow of heavy overload currentsin said circuit.

7. A thermal device comprising a current carrying thermal element havinga slot extending inwardly from one edge to form a U-shaped element, thebase of said slot being enlarged to provide a substantially circularaperture, and a pin of conducting material extending through saidaperture, said pin being of a diameter to fit tightly in said apertureso as to provide a conducting bridge between the legs of said U-shapedelement and thereby prevent excessive concentration of current in saidthermal element adjacent the base of said slot upon the flow or heavyoverload currents in said element.

8. A thermal device comprising a current carrying thermal element havinga slot extending inwardly from one edge to form a U-shaped element, thebase of said slot being enlarged to provide a substantially circularaperture, and a rivet formed of conducting material tightly fitting insaid aperture so as to provide a conducting bridge between the legs ofsaid U-shaped element and thereby prevent excessive concentration ofcurrent in said thermal element adjacent the base of said slot upon theflow of heavy overload currents in said element.

9. A thermal device comprising a current carrying bimetallic elementhaving a slot extending inwardly from one edge to form a U-shapedelement, the base of said slot being enlarged to provide a substantiallycircular aperture, and a conducting pin'extending through said aperture,said pin and the edges of said aperture being provided with a coating ofhighly conducting material so as to provide a low resistance connectionbetween the legs of said U-shaped element and thereby prevent excessiveconcentration of current in said element adjacent the base of said slotupon the flow of heavy overload currents in said element.

10. A thermal device comprising a U-shaped current carrying bimetallicelement, a conductor fixedly secured to the legs of said elementadjacent the yoke portion thereof for electrically connecting said legsin shunt to said yoke, the

, engaging portions of said element and said conductor being providedwith a coating of highly conducting material so as to provide a lowresistance connection between the legs of said U- shaped element andthereby prevent excessive concentration of current in the inner edge ofsaid yoke upon the flow of heavy overload cur rents in said element.

. KURT KOCH.

